1. Technical Field
The present invention relates generally to the field of semi-conductor manufacturing and, more specifically, to a method for optimizing the use of x-ray lithography in conjunction with hybrid resists.
2. Background Art
The need to remain cost and performance competitive in the production of semiconductor devices has caused continually increasing device density in integrated circuits. To facilitate the increase in device density, new technologies are constantly needed to allow the feature size of these semiconductor devices to be reduced. For the past 20 years, optical lithography has driven the device density and the industry has resorted to optical enhancements to allow increasing densities. As an example, some such enhancements include overexposing/overdeveloping, hard and soft phase shifts, phase edge masks, and edge shadowing. Unfortunately, the latest of such enhancements tend to offer only minor increases in density and the limit of optical enhancements appears inevitable in the near future.
The same industry trend to increase device density is also causing a transition to x-ray lithography. X-rays are a desirable type of exposure radiation because the wavelength of x-rays (about 8 .ANG.) is smaller than the wavelength of ultra-violet radiation typically used to fabricate dense integrated circuits. The smaller wavelength allows for exposure of a resist through a mask having a smaller image than in optical lithography. However, certain aspects of x-ray lithography are insufficiently advanced to satisfy the current demand for increasing device density.
In optical lithography, projection printing allows the mask image to be projected onto the resist at a reduced size to increase the possible device density. For example, a 4.times. reduction is typical. However, sufficient success has not been achieved in attempts to fabricate lenses for reducing the size of the x-ray mask image to a smaller aerial image. Accordingly, even though the potential exists for exposing a 0.05 .mu.m aerial image onto the resist using x-ray lithography, it requires a mask having a 0.05 .mu.m image. Producing such a mask within required tolerances can be a formidable task and constitutes the most significant disadvantage of x-ray lithography. In fact, the tolerances that are achieved in the x-ray mask dictate the tolerances that can be achieved in a product produced using x-ray lithography.
The smaller images on a x-ray mask are more difficult to fabricate than the larger images on an optical mask because of the smaller image size and different materials. The smaller size requires electron beam lithography (EBL) to carve out the image and EBL has not yet advanced sufficiently to produce masks that take full advantage of the smaller wavelength. Further, instead of a chrome mask layer used in optical lithography, the nature of x-rays requires tungsten, gold, or other material with a high x-ray extinction coefficient that must also be much thicker than the typical chrome layer. Unfortunately, the high x-ray extinction materials are difficult to control within tolerance and the thickness increases the difficulty of mask fabrication.
Also, x-ray lithography involves proximity printing the mask image onto the resist. Proximity printing simply means that the mask is in close proximity to, but not in contact with, the surface of the resist layer. The gap distance between the mask and the wafer is minimized to produce an aerial image through the mask with as high a contrast possible. That is, the gap distance is decreased so that the transition from zero intensity to full intensity occurs over a smaller area. Typical gap distances are between 10 and 50 .mu.m.
The pitch, or combined width of an adjoining line and space in a semiconductor, can theoretically be very small when x-ray lithography is used, but the poor tolerance of the x-ray mask prevents the precise formation of reliable devices at x-ray pitch. Even though the possibility of x-ray pitch exists, abnormalities in the x-ray mask will yield abnormalities in lines and spaces sufficient to preclude fabricating reliable devices as small as allowed by the small x-ray wavelength. It would be an improvement in the art to provide a method for forming high tolerance devices with x-ray pitch. Such a method must yield few enough abnormalities in lines and spaces to provide performance of the final product within industry standards. Without a method for forming high tolerance devices at x-ray pitch, the value of x-ray lithography for increasing device density is seriously diminished and advancement in improving chip cost and performance may stagnate.